The primary focus of this proposal is the genetic organization and function of the Y chromosome of Drosophila melanogaster. The Y chromosome is unique; it is required for male fertility and its function is apparently restricted to the primary spermatocyte, one cell cycle in one sex in the life cycle of the species; it is a large chromosome with little more than six male-fertility genes separated from one another by large blocks of simple sequence DNA. The products of three of these species have been identifies as high-molecular weight polypeptides that are structural components of the sperm axoneme. Deficiencies for two of the three also eliminate specific structures from primary spermatocyte nuclei. We propose to characterize these genes and their products. We propose to clone them either by means of by transposon tagging with P elements in male-sterile Y's recovered from dysgenic crosses, by walking from previously cloned autosomal region in a male-sterile Y-autosome translocation with a breakpoint in region, or by screening testis cDna libraries. For any fertility gene successfully cloned we intend to examine its sequence organization and that of its environs, and we propose to examine the intercellular and interacellular distribution of its product during spermatogenesis by immunocytochemical procedures. Deficiencies for k1-3 and k1-5, the distal-most fertility gene of YL lack aggregates of reticular and tubular elements respectively, which are normally present in primary spermatocyte nuclei. We prospoe to investigate the chromosomal relations between spermatocyte-nuclear structures and male fertility by the genetic fractionation of the distal portion of YL.